Treatment with appropriate combinations of antiretroviral drugs can reduce HIV-1 viremia to below the limit of detection and allow reconstitution of the immune system. The correct choice of antiretroviral drugs is critical for achieving and maintaining suppression of viral replication. Interestingly, there is no widely accepted system for comparing the antiviral activity of different drugs. Previous studies from this lab have shown that the inhibitory potential of antiretroviral drugs is strongly dependent upon a previously ignored factor termed here the slope parameter. This parameter describes the steepness of the dose-response curve. The inhibitory potential of antiretroviral drugs, described in a new index developed by this lab, varies for different classes of antiretroviral drugs by over 10 logs (10,000,000,000 fold!) in a manner that is strongly influence by the slope parameter. Thus the slope parameter is a critical missing dimension in the analysis of the suppressive potential of antiretroviral drugs. This proposal seeks to understand the molecular mechanisms underlying this effect and to use that information to guide the development of drugs and vaccines that will maximally suppress viral replication. The first specific aim is to test a mechanistic hypothesis involving a unique form of intermolecular cooperativity which can explain the shapes of dose-response curves for antiviral drugs. This hypothesis will be tested in a very specific way using phenotypic mixing experiments with viruses carrying both wild type and mutant forms of the proteins targeted by the drugs. The second aim is to understand the high magnitude and drug-to-drug variability of the slope parameter for the protease inhibitor class of antiretroviral drugs. Among the drug classes tested to date, the protease inhibitors show the highest slope values. As a result, clinical concentrations of some of these drugs can inhibit single round infections by ~10 logs. This project seeks to identify the mechanism underlying this extraordinary susceptibility of HIV-1 to inhibition by some protease inhibitors. The third specific aim is to apply this method of analysis to novel classes of antiretroviral drugs. Since a high slope value is critical for achieving multi-log inhibition of single round infection, the proposed analyses could identify drug classes that are likely to be particularly effective in vivo. The fourth aim is to determine the effect of drug resistance mutations on dose-response curve slope. Correct prediction of the clinical consequences of drug resistance mutations requires an understanding of the effects of the mutations on the slope parameter. The final aim is to measure the slope parameter of neutralizing antibodies directed at the HIV-1 envelope protein since concept of dose-response curve slope also applies to vaccine-induced effector mechanisms including neutralizing antibodies.